1. Field of the Invention
The present invention relates to an SOI (Semiconductor On Insulator) type thin-film transistor, and more particularly to a technique for fixing a potential of its body (referred to as "body potential" hereinafter).
2. Description of the Background Art
FIG. 18 is a cross-sectional view illustrating a structure of a general SOI type thin-film transistor 900. The thin-film transistor 900 is formed as an n-channel MOS transistor in a semiconductor layer 902 provided on an insulator 901. The insulator 901 may be formed as a buried layer in a not-shown semiconductor substrate.
In the p.sup.- type semiconductor layer 902, a source region 903 and a drain region 904 both of which are n.sup.+ type semiconductor layers are provided at a distance from each other. The semiconductor layer 902 sandwiched by the source region 903 and the drain region 904 is termed "body" of the thin-film transistor 900. Above the body, a gate electrode 907 made of e.g., polysilicon is provided with a gate insulating film 906 interposed therebetween.
Since the semiconductor layer 902 is provided on the insulator 901, its body potential is in a floating state in the structure of FIG. 18. In this state, there are possibilities of generation of leak current and unstable operation of the thin-film transistor 900 due to variations in power-supply level and ground level and parasitic bipolar effect. The parasitic bipolar effect here refers simply to a phenomenon that positive holes created by impact ionization are accumulated in the body and the body potential thereby rises to increase a leak current in the thin-film transistor 900 as an n-channel MOS transistor.
There may be a case where a cosmic ray such as an .alpha. ray enters the body to form a pair of an electron and a positive hole. Since the thin-film transistor 900 is used in a state where a channel is formed by inverting a surface of the body into n type, the positive hole is accumulated, though the electron is drawn out, to raise a possibility of inviting a rise of body potential.
To solve the above problem, a technique for fixing the body potential has been proposed. FIG. 19 is a cross-sectional view of a first technique in the prior art, illustrating a structure of a thin-film transistor 800 and a structure for fixing its body potential. The thin-film transistor 800 comprises a p.sup.- type semiconductor layer 802 as a body formed on an insulator 801 and a source region 803 and a drain region 804 both of which are of n.sup.+ type and provided thereabove. Above the body, a gate electrode 807 is provided with a gate insulating film 806 interposed therebetween.
Alongside the thin-film transistor 800, an isolation oxide film 809 is formed by LOCOS oxidization of the semiconductor layer 802. Below the isolation oxide film 809, above the insulator 801, a region 805a is formed by enhancing the conductivity of the semiconductor layer 802. On the opposite side of the thin-film transistor 800 with respect to the isolation oxide film 809, a p type region 805b and a p.sup.+ type region 805c are layered on the insulator 801 in this order. The regions 805a, 805b and 805c adjoin the semiconductor layer 802 in this order, and when a potential VB is applied to the region 805c, the body potential can be fixed at a position away from the thin-film transistor 800 with the isolation region 809 interposed.
Since the first background-art technique, however, uses the isolation oxide film 809, it is not suitable for integration. Further, a structure much like that of FIG. 19, where the p type semiconductor layer is provided between the source region and the insulator to draw the positive hole, is disclosed in, for example, Japanese Patent Application Laid Open Gazette No. 6-232405.
On the other hand, the thin-film transistor is often used with the potential applied to the source region (referred to simply as "source potential") and the body potential being equal, and on the premise of such a use, a structure for fixing the body potential can be formed locally in the source region. FIG. 20 is a plan view illustrating a structure of a thin-film transistor 700 that is advantageous from this viewpoint. The second technique in the background art is disclosed in, for example, "Silicon-on-insulator technology: materials to VLSI" by J. P. Colinge (Kluwer Academic Publishers, 2nd Ed.).
With the gate electrode 707 centered, on the left hand of this figure provided are an n.sup.+ type source region 703 and p.sup.+ type body-potential drawing regions 705a and 705b which sandwich the region 703 vertically in this figure, and on the right hand of this figure provided is a drain region 704. A contact structure for supplying the body potential and the source potential is formed at a contact region 310 provided covering part of the body-potential drawing regions 705a and 705b across the source region 703. This structure eliminates the necessity of the LOCOS oxide film used in the first background-art technique, thereby being suitable for integration.
The second background-art technique, however, has great problems as follows. The first problem is due to the position of the body-potential drawing region 705a. The body is provided in the back of the gate electrode 707 in this figure, though not shown, and a channel is formed mainly in a portion surrounded by the source region 703, the drain region 704 and the gate electrode 707. From this portion, the positive hole should be drawn.
In the structure of FIG. 20, the body-potential drawing regions 705a and 705b are positioned at an end of the source region 703 along a direction where the gate electrode 707 extends (in a vertical direction of this figure). Therefore, in order to effectively draw the positive hole from the body, a pair of body-potential drawing regions 705a and 705b are needed. For example, if the body-potential drawing region 705a is not provided, the body-potential drawing region 705b can not effectively draw the positive hole from a portion on the upper side of this figure in the body. This needs a larger area for the body-potential drawing regions 705a and 705b, and a portion which does not function as a channel in a direction (gate width) where the gate electrode 707 extends increases in width. That inhibits integration of the thin-film transistor.
The second problem becomes pronounced in a case where the gate electrode 707 is made of polysilicon and the like. Impurity implantations for forming the source region and the drain region are performed, in general, by using the gate electrode and the gate insulating film provided between the gate electrode and the body as a mask in a self-aligned manner. When an impurity to be implanted into the polysilicon to enhance the conductivity as the gate electrode is equivalent in conductivity to those for the source region and the drain region, the conductivity of the gate electrode is obtained by impurity implantation for forming the source region and the drain region.
If a p type impurity is implanted to also form the p.sup.+ type body-potential drawing regions 705a and 705b of FIG. 20 in a self-aligned manner, however, an effect of the n type impurity which the gate electrode 707 has is counter-doped. The gate electrode 707 comprises a straight portion 707b which is straight in a direction where the gate electrode 707 extends and a contact portion 707a in which a contact structure is formed to apply a predetermined electrical signal to the gate electrode. The conductivity is degraded in portions 401a and 401b of the straight portion 707b near the p.sup.+ type body-potential drawing regions 705a and 705b at an end (on the left hand of FIG. 20) in a direction horizontal direction of this figure) orthogonal to the direction where the straight portion 707b extends. This phenomenon becomes especially pronounced in the portion 401b near the contact portion 707b because transmission of signals to the straight portion 707b is degraded.
Even if a mask is used to selectively implant the p type impurity to form the p.sup.+ type body-potential drawing regions 705a and 705b out of the self-aligned manner, a margin for alignment of the mask is needed in order to surely bring the body-potential drawing regions 705a and 705b into contact with the body, and implantation of the p type impurity into the gate electrode 707 can not be virtually avoided.